1. Field of the Invention
This invention relates to a method and system for producing methyl methacrylate. In one aspect, this invention relates to the purification of methyl methacrylate. In another aspect, this invention relates to the recovery of methanol in the production of methyl methacrylate. In another aspect, this invention relates to the use of membrane technology for purification of methyl methacrylate. In yet another aspect, this invention relates to the use of molecular sieve membranes for purification of methyl methacrylate.
2. Description of Related Art
Methyl methacrylate (MMA) is by far the most important methacrylic acid ester and is widely used for producing acrylic plastics, e.g., polymethyl methacrylate, and polymer dispersions for paints and coatings. In 2008, over 20% of the world consumption of MMA was MMA produced in the United States. According to a recent report, the global MMA market will reach 2.9 million metric tons by 2015. In a typical industrial process for producing MMA, a crude mixture containing MMA, methanol, and water is produced. Conventionally, separation of the mixture components is currently carried out using distillation. See, for example, U.S. Pat. No. 5,028,735 which teaches purification and preparation processes for methyl methacrylate in which a mixture containing water, methanol, and MMA as primary components along with at least one of methyl acrylate, methyl propionate, and methacrylic acid is azeotropically distilled together with hexane to obtain MMA in a form substantially free of water, methanol, methyl acrylate, methyl propionate and hexane and a low boiling point fraction, the latter of which is cooled and separated into a water phase and an oil phase. Methanol is recovered from the water phase by adding an alkaline substance and then distilling the resultant mixture. See also U.S. Pat. No. 5,435,892 which teaches a distillation process for separating methanol from a mixture of methanol with methyl acrylate or methyl methacrylate, as well as from a mixture of methanol and water with methyl acrylate or methyl methacrylate with the use of an azeotropic solvent, which forms an azeotropic mixture with methanol. One of the drawbacks of conventional distillation for separating and purifying MMA is that it is difficult and energy-intensive because the boiling point of MMA is close to that of water and methanol, as a result of which the MMA forms an azeotrope with them. The three-stage distillation technology currently employed in the industry for MMA purification requires a substantial amount of energy.
FIG. 1 shows an industrial three-column design flowsheet for a conventional three-stage azeotropic distillation process for MMA purification.